Before chromium was recognized as environmentally hazardous, compositions containing chromium were used as coatings for improving the corrosion resistance and paint adhesion of metals. Chromium chromate and chromium phosphate were two compounds typically used as coatings for metal. One advantageous side effect of these compounds was that they left a slight coloration on the coated metal, thereby assuring the user that the coating was in fact applied to the metal.
Recent developments led to the use of chrome-free coatings, or "pretreatments," which are typically based on the fluo or oxy-fluo forms of group IV-B elements of the periodic table (e.g., titanium, zirconium, and hafnium). These chrome-free pretreatments provide surface coatings whose compositions are based on the products of the interaction between the metallic substrate and these group IV-B compounds.
While chrome-free pretreatments have been commercially successful, one of the major objections to this technology has been that chrome-free pretreatments leave little or no color on the metal, unlike the previously used chromium compositions. Thus, the only indication to the user that the pretreatment has been applied to the metal is surface appearance characteristics, such as uniformity of wetting and a viscous fluid surface film. Even at extreme deposition (i.e. high coating weight) of chrome-free pretreatments, where a slight indication of coloration is present, the coloration is so faint that it is of no practical use. Accordingly, because chrome-free pretreatments show little or no coloration, the user has no option other than analytical methods to determine the presence and the amount of pretreatment on the metal surface.